A twin-screw degassing extruder is described in U.S. Pat. No. 4,423,960. In such arrangement, individual screw sections are mounted on a spindle. The screw sections are prevented from rotation relative to one another by key members which engage in a groove in the spindle.
The internal diameter of the barrel of the twin-screw extruder disclosed in this prior document is enlarged in the degassing region in order to obtain a greater free volume and hence to achieve better degassing of the plastics material melt.
These structural features do give good degassing results. On the other hand, the enlargement of the barrel in the degassing region is a very cost-intensive feature in the production of the barrels. This is because it is necessary to produce barrels and screw sections having enlarged diameters in the degassing region, which diameters differ from the respective diameters of the barrel and screw in the input region of the extruders.
In such known prior art extruders, the screw sections have helical flights disposed around their peripheries in the region below the degassing port. These flights occupy a large volume of the interior of the barrel. The conveying operation of an extruder is, in general, achieved by the adhesion of the melt to the interior wall of the barrel with substantially simultaneously the melt being scraped from the wall by the helical flights provided on the rotating screw. It is generally true to say that the better the material adheres to the interior wall of the barrel, the better the conveying effect of the helical flights of the screw.
However, since a considerable portion of the internal surface of the barrel, in which the material is being conveyed, is missing due to the provision of the degassing port, the conveyance of material is inadequate in the degassing region of such known degassing extruders. The material remains in the screw threads as a block or undergoes laminar flow without performing a rolling movement against the internal wall of the barrel if, as is necessary, part of the internal wall is absent as a result of the provision of the degassing port.